Telescopic cylinders push rather than dump

Telescopic cylinders are often the only choice for mobile equipment requiring high forces and long actuation lengths, but with little room available. Philippi-Hagenbuch Inc., Peoria, Ill., uses three-stage telescopic hydraulic cylinders in their off-highway rear eject truck bodies and trailers. These versatile vehicles perform the same function as conventional dump trucks, but hold several advantages:

material can be dumped in areas with low overhead clearance,

material can be distributed more effectively because ejecting the load is more controllable than relying on it to fall out (often in large clumps),

trucks can be dumped on unstable ground (raising the body of a conventional truck on ground that is unstable or not level creates the potential for the truck tipping over),

material can be stockpiled because the rear-eject unit can eject the load up hill, and

material that otherwise would stick to a conventional truck dump body can be forced from a rear eject unit.

Hydraulic operation

The key element of PhilippiHagenbuch's ejectors is a three-stage telescopic cylinder from Prince Mfg. Corp., North Sioux City, S.D. A second cylinder opens and closes the tailgate, and both cylinders are controlled through a single manifold.

Load ejection is under complete control of the vehicle operator. Typically, the operator pulls a lever, which routes hydraulic fluid to a double-acting, three-stage telescopic cylinder. The cylinder pushes an ejector blade that ejects the load from the back of the truck or trailer bed. A mechanical linkage between the cylinder and the tailgate controls opening and closing of the tailgate automatically.

During extension, a regenerative circuit routes hydraulic fluid from the return line of the cylinder into the extension line. The combined flow from the hydraulic pump and the return line speeds cylinder extension until a predetermined pressure is reached. When this occurs, return fluid flows to tank, and full hydraulic force is available to eject the load. As with most regenerative circuits, the primary benefit is reduced cycle time, which boosts productivity.

Hydraulics was chosen for this application because no other method could produce the high force and long stroke required — especially in such a small space. A hydraulic integrated circuit custom-designed by Prince provides multiple control functions. Furthermore, no electronics are necessary because hydraulics provides effective and reliable power and control.

A horizontal challenge

Telescopic cylinders used in conventional dump trucks are oriented vertically. Therefore, the weight of the dump body and the cylinder itself may provide enough force to fully retract the cylinder. Furthermore, vertical orientation means that the majority of the cylinder's weight acts longitudinally.

In contrast, Philippi-Hagenbuch's telescopic cylinder is mounted horizontally. This means the cylinder must be double acting because gravitational force does not act in a direction to retract the cylinder. Furthermore, the weight of the cylinder acts radially, rather than axially. Left unchecked, this radial force could produce side loads on the cylinder that could accelerate rod and bearing wear. Therefore, the cylinder contains oversized rod bearings to prevent premature wear.

More importantly, horizontal orientation can cause cylinder stages to extend out of sequence. When this occurs, internal flow paths can be blocked, causing the cylinder to stall. To prevent this, Prince supplies Philippi-Hagenbuch with an alternative design with flow paths through tubes, rather than through ports on the sides of each stage.